You can do it in your head:

The bus is 100 miles down the road when the car starts, and the car approaches
the bus at the rate of 10mph, their difference in speeds, so the car can make
up the bus's 100 mile head start in 10 hours.

Or by algebra, the way your teacher probably wants you to do it:

Make this chart

       Distance      Rate      Time
Bus    
Car    

Let x be the car's time.

       Distance      Rate      Time
Bus    
Car                              x

The bus traveled 2 hours more than the car, so
the bus's time is x+2


       Distance      Rate      Time
Bus                             x+2
Car                              x

Fill in the two rates:
       Distance      Rate      Time
Bus                   50        x+2
Car                   60         x

Now Use Distance = Rate × Time


       Distance      Rate      Time
Bus     50(x+2)       50        x+2
Car       60x         60         x

The two distances are the same when the car catches the bus, so
we set the two distances equal:

              50(x+2) = 60x
              
Solve that and get x = 10 hours, the same as what we get doing it in our 
heads.

Edwin